Roommates

"There's someone inside the cubicle!" When he saw the black leather shoes, Chen Ge was instantly reminded of Mr. Bai from the eastern campus. The latter was wearing a pair of leather shoes just like that. "Stay back."

Chen Ge took two steps back. He took out Lin Sisi's phone, lowered it to the gap, and snapped a quick picture.

Lowering his head to look at the phone, the thing that Chen Ge worried about did not happen. There was only a pair of shoes inside the cubicle; there was no one wearing the shoes.

Lin Sisi's phone can capture the image of ghosts and other unique entities. Since there is nothing on the picture, this can only mean that the shoes are the only thing inside the cubicle. But if the cubicle is unoccupied, why is the door locked from inside?